This invention relates generally to the chemical-mechanical polishing (CMP) of metal substrates on semiconductor wafers and slurry compositions therefor. In particular, the present invention relates to a stable CMP slurry composition containing a low concentration of abrasive particles. Particles of fumed silica were found to be helpful in stabilizing the hydrogen peroxide contained in the slurry composition when the slurry is stored prior to its use. An extension of this concept is to create a complete, ready-to-use, fumed silica CMP slurry that is stable over long-term storage at ambient conditions.
Silicon based semiconductor devices, such as integrated circuits, typically include a silicon dioxide (SiO2) dielectric layer. Multilevel circuit traces, typically formed from aluminum or an aluminum alloy, are patterned onto the SiO2 substrate.
If the aluminum based circuit traces are replaced with copper based circuit traces, the density of circuit traces on the face of the device could be increased because copper has a higher electrical conductivity than aluminum enabling the use of circuit traces with a reduced cross-sectional area. In addition, the electromigration of copper is approximately 0.1 that of aluminum at a given temperature.
In order to use multilevel metals circuit traces, lithographic focus constraints require that the substrate surface be planar on both a global and local scale. If the surface is not planar, the exposure tool cannot be focused properly causing out of focus images and poor quality printing.
One way to fabricate planar copper circuit traces on a silicon dioxide substrate is referred to as the damascene process. In accordance with this process, the silicon dioxide dielectric surface is patterned by a conventional dry etch process to form holes and trenches for vertical and horizontal interconnects. The patterned surface is coated with an adhesion-promoting layer such as titanium or tantalum and/or a diffusion barrier layer such as titanium nitride or tantalum nitride. The adhesion-promoting layer and/or the diffusion barrier layer is then overcoated with copper. Chemical-mechanical polishing is next employed to reduce the thickness of the copper overlayer, as well as the thicknesses of any adhesion-promoting layer and/or diffusion barrier layer, until a planar surface that exposes elevated portions of the silicon dioxide surface is obtained. The vias and trenches remain filled with electrically conductive copper forming the circuit interconnects.
Previously, it was believed that the removal rate of the copper and the adhesion-promoting layer and/or the diffusion barrier layer must both greatly exceed the removal rate of silicon dioxide so that polishing effectively stops when elevated portions of the silicon dioxide are exposed. The ratio of the removal rate of copper to the removal rate of silicon dioxide base is called xe2x80x9cselectivity.xe2x80x9d When high selectivity copper slurries are used, the copper layers are easily over-polished creating a depression or xe2x80x9cdishingxe2x80x9d effect in the copper vias and trenches. This feature distortion is unacceptable due to lithographic and other constraints in semiconductor manufacturing.
Another feature distortion that is unsuitable for semiconductor manufacturing is called xe2x80x9cerosion.xe2x80x9d Erosion is the topography difference between a field of silicon oxide and a dense array of copper vias or trenches. In chemical-mechanical polishing, the materials in the dense array are removed or eroded at a faster rate than the surrounding field of silicon oxide. This causes a topography difference between the field of silicon oxide and the dense copper array. The industry standard for erosion is typically less than 500 Angstroms (xc3x85).
A typically used chemical-mechanical polishing slurry has two actions, a chemical component and a mechanical component. An important consideration in slurry selection is xe2x80x9cpassive etch rate.xe2x80x9d The passive etch rate is the rate at which copper is dissolved by the chemical component alone and should be significantly lower than the removal rate when both the chemical component and the mechanical component are involved to prevent under-cutting of copper contained in the trenches and vias by contact with the chemical component. A large passive etch rate leads to dishing and thus, preferably, is less than 10 nanometers per minute.
A number of systems for chemical-mechanical polishing of copper have been disclosed. Kumar et al. in an article entitled xe2x80x9cChemical-Mechanical Polishing of Copper in Glycerol Based Slurriesxe2x80x9d (Materials Research Society Symposium Proceedings, 1996) disclose a slurry that contains glycerol and abrasive alumina particles. An article by Gutmann et al. entitled xe2x80x9cChemical-Mechanical Polishing of Copper with Oxide and Polymer Interlevel Dielectricsxe2x80x9d (Thin Solid Films, 1995) discloses slurries based on either ammonium hydroxide or nitric acid that may contain benzotriazole (BTA) as an inhibitor of copper dissolution. Luo et al. in an article entitled xe2x80x9cStabilization of Alumina Slurry for Chemical-Mechanical Polishing of Copperxe2x80x9d (Langmuir, 1996) discloses alumina-ferric nitrate slurries that contain polymeric surfactants and BTA. Carpio et al. in an article entitled xe2x80x9cInitial Study on Copper CMP Slurry Chemistriesxe2x80x9d (Thin Solid Films, 1995) disclose slurries that contain either alumina or silica particles, nitric acid or ammonium hydroxide, with hydrogen peroxide or potassium permanganate as an oxidizer.
There are a number of theories as to the mechanism for chemical-mechanical polishing of copper. An article by Zeidler et al. (Microelectronic Engineering, 1997) proposes that the chemical component forms a passivation layer on the copper changing the copper to a copper oxide. The copper oxide has different mechanical properties, such as density and hardness, than metallic copper and passivation changes the polishing rate of the abrasive portion. The above article by Gutmann et al. discloses that the mechanical component abrades elevated portions of copper and the chemical component then dissolves the abraded material. The chemical component also passivates recessed copper areas minimizing dissolution of those portions.
While present day chemical-mechanical polishing systems are capable of removing a copper overlayer from a silicon dioxide substrate, the systems do not satisfy the rigorous demands of the semiconductor industry. These requirements can be summarized as follows. First, there is a need for high removal rates of copper to satisfy throughput demands. Secondly, there must be excellent topography uniformity across the substrate. Finally, the CMP method must minimize local dishing and erosion effects to satisfy ever increasing lithographic demands.
Presently, for metal CMP slurries to work, additions of certain chemical agents to increase the removal rates of the metal layers are necessary. ARCH Wacker has developed a basic slurry to do the CMP of the copper, tantalum and TEOS layers. This is described in a publication titled xe2x80x9cDevelopment of a 1:1:1 Slurry for Tantalum Layer Polishingxe2x80x9d in the proceedings of the CMP-MIC conference, February 1999. This prior art slurry was made by mixing four components together; organic acid, inorganic oxidizer, aqueous abrasive dispersion, and corrosion inhibitor. These chemicals are required for purposes such as Cu corrosion prevention and the pH adjustment of the slurry to achieve the required rates on various metallic and non-metallic layers of the wafers. These components could, in theory, be combined in a dilute aqueous solution, which could be added to an aqueous dispersion containing abrasive particles, to form the finished slurry.
Prior art CMP metal slurries are typically two part mixtures consisting of a dispersion and an oxidizer. The dispersion comprises an abrasive, an acid to lower the pH to about 2 to 6, optionally a surfactant which maintains the abrasive in suspension and other chemicals tailored to the metal being polished. An example is a tungsten layer slurry called Biplanar(copyright) made by EKC. The dispersion is an acidic dispersion (approximately pH of 3, with 5 to 15% alumina particles). Acids reportedly used in the slurry include carboxylic acids or nitric acid. At the point of use, the dispersion is mixed with an oxidizer, such as hydrogen peroxide or ferric nitrate, to form the slurry that will be used to polish the metal layers.
Metal slurry manufacturers typically sell only the acidic dispersion while the customer buys the oxidizer independently and mixes the two parts at the point of use. In this case, the oxidizer is a standard bulk commodity solution that can be mixed with the different customized metal dispersion solutions. For copper CMP, the inorganic oxidizer is usually H2O2. In practice, the combination of the particular ingredients for the copper slurry described results in a solution which does not maintain its properties over a sufficiently long time (i.e., shelf life is short). In particular, it was found that the H2O2 was decomposing. This has the end result of changing the polishing performance of the slurry, resulting in an unstable process for the end user, and also creating a dangerous outgassing potential which could rupture sealed storage containers. In addition, it required the individual chemical components of the CMP slurry to be shipped and stored in separate containers until the slurry was made just prior to use. If the manufacturer does not forecast appropriately and the dispersion reaches its shelf life, then a large volume of dispersion may have to be disposed which is very expensive and environmentally unfriendly, causing additional complications to the end user.
The present invention provides a complete, ready-to-use CMP slurry that is stable over long term storage at ambient conditions. In accordance with the invention, there is provided a CMP slurry for polishing a substrate. The slurry comprises abrasive particles and an oxidizing agent, wherein the slurry exhibits a stability having a shelf life of at least 30 days.
The invention also provides a method to use this slurry in a CMP process that promotes high removal rates, low defect densities and reduced amounts of dishing and erosion.